1. Technical Field
This invention relates to sand core making by hot box techniques, and more particularly to full curing of such cores within the hot box before removing the core.
2. Discussion of the Prior Art
There are two basic methods using resin bonded sand in cores, the cores being used in subsequent metal casting operations. First there is the cold box core making method in which polyurethane resin binders are mixed with the sand and the mixture cured by infusion of catalyst gases into the core box to polymerize the binder. Secondly, there is the hot box core making method which uses a starting sand mixture comprised of resin binder and a liquid catalyst, the mixture being blown into the interior of the core box and then triggered to polymerize by the use of exteriorly applied heat. Heat is conducted from the outer regions of the sand core to the interior regions and although the curing action takes place at temperature as low as 120.degree. F., it is necessary to achieve temperatures of 450.degree. to 550.degree. F. to stimulate proper polymerization of the sand core within a short period of time, such as 20 to 40 sec. (depending upon size and shape of core). The sand cores must be removed prematurely from the core box possessing only a fully cured outer skin with a partially cured interior core which must be fully cured in a separate independent core furnace, if required. Only in this manner has the hot box core making method been adopted to rapid high volume production. The exterior applied heat is provided by gas burners impinging on the box, often possessing flame temperatures of about 1600.degree. F. Because the heat is so intense, large massive sand cores can only be cured rapidly to a very shallow skin depth, 1/4 to 1/2 inch deep, while the rest of the core remains uncured. There is a high risk of damage and distortion in moving such cores to and during subsequent furnace curing (post curing). In both stages of curing, heat is transferred from the surface of the core to its center by conduction only. As sand is a good insulator, the process is energy intensive. Hot box cores continue to cure after they are removed from the core box due to exothermic reaction. Formaldehyde, a product of the curing reaction, is given off directly to the manufacturing area. Also, there is a tendency for excessive core box temperatures to burn the cores at the surface and thereby cause scrap.
In spite of such drawbacks, hot box core making is desirable because of its low cost, potential for high productively, and the relative quality of the cores in high volume production. To make such technology even more efficient, it would be desirable to quickly carry out complete curing of the sand cores within the hot core box prior to removal of the core, such as by convection heating in addition to conductive heating. Conventional hot box designs present three obstacles to providing a solution to this problem: (i) to introduce heat more rapidly and uniformly through the depth of the core, such as by convection heating, the box must be sealed; boxes do not contain resilient sealing today because of the very high heat of burner impingement directly on the metal core box; (ii) core boxes for large massive cores must be horizontally parted because of the need to extract the core, without damage, by giving support in at least to the lower part during removal, but such parting interferes with convection heating as well as with surrounding burners; and (iii) the adhesion between the sand core and the hot box, after treating, necessitates the use of mechanical ejector pins carried by movable plates, such ejector pins presenting additional sealing problems if convection heating is to be used.